In present-day seismic exploration, there are several methods for acquiring data in a marine environment. The most common method is to use a marine vessel to tow a line of seismic acoustic pulse detectors behind a line of acoustic pulse generators. Unfortunately, this type of arrangement does not permit extremely long spacing between the line of acoustic pulse generators and acoustic pulse detectors. In a system where a line of acoustic pulse detectors is towed behind the acoustic pulse generator, reflection data is obtained. Reflection data is that data based upon returning acoustic waves that are reflected back from a subsurface interface or change in density. By extending the spacing between seismic pulse generators and seismic pulse detectors, refraction data may also be obtained. Refraction data is that data based upon acoustic waves that are returning back from the source side of an interface after traveling along the subsurface interface.
A method for acquiring marine seismic data which permits extended spacing between seismic pulse generators and seismic pulse detectors is the use of an on-bottom seismic cable. A seismic cable containing a plurality of hydrophones, spaced apart periodically, is placed on the floor of the ocean or a similar body of water with marker buoys at each end. A marine vessel towing a string of air guns or other acoustic pulse generators sails across lines having predetermined locations with respect to the on-bottom cable. Depending on the spacing between the acoustic pulse generators and the on-bottom cable, reflection or refraction data may be obtained. By extending the spacing between a seismic pulse source or generator and seismic pulse receivers, refraction and reflection data may be obtained.
More detailed information concerning the construction of an ocean bottom seismic cable can be found by referring to copending patent application Ser. Nos. 579,041 now abandoned, 579,042 now abandoned and 579,043 now abandoned, titled "Strain Member Chassis", "On Bottom Cable Termination" and "Jacketed Cable Section", respectively, all assigned to the present assignee.
Although an ocean bottom cable has a marker buoy at each end, the length of the cable is several miles and much irregular terrain lies between the buoys. As such, the cable will have a depth variance and a variance from a straight line connecting the two marker buoys. The depth of the ocean bottom cable can be indicated by depth detectors which may be spaced along the length of the cable. The actual location of the acoustic pulse detectors is unknown because the deviation from the straight line connecting the buoy locations is not determined and can be as great as several hundred meters.
A method for locating ocean bottom cable is taught in copending patent application Ser. No. 605,089 now U.S. Pat. No. 4,641,287, filed Apr. 30, 1984, titled "Method For Locating An On-Bottom Seismic Cable", assigned to the present assignee and incorporated herein by reference in its entirety. That application discloses a method for determining the exact position of an ocean bottom seismic cable. A plurality of spaced-apart shots are fired from an acoustic pulse source. A response to each of the shots is produced by an acoustic pulse detector on the ocean bottom cable. The travel time of each shot implies a distance between the source and detector. The distance may be used as a radius defining a spherical surface upon which the detector is located. The intersection of the spheres defined by successive shots define a circumference upon which the detector must be located. The intersection of a third sphere with the circumference thus defined produces two distinct possible locations of the detector. Depth detectors are used to control the possibilities and indicate the exact location of the detector.